Where this interface is implemented as a singleton class such as
SingletonBeanFactoryLocator, the Spring team strongly
suggests that it be used sparingly and with caution. By far the vast majority
of the code inside an application is best written in a Dependency Injection
style, where that code is served out of a
BeanFactory/ApplicationContext container, and has
its own dependencies supplied by the container when it is created. However,
even such a singleton implementation sometimes has its use in the small glue
layers of code that is sometimes needed to tie other code together. For
example, third party code may try to construct new objects directly, without
the ability to force it to get these objects out of a BeanFactory.
If the object constructed by the third party code is just a small stub or
proxy, which then uses an implementation of this class to get a
BeanFactory from which it gets the real object, to which it
delegates, then proper Dependency Injection has been achieved.

As another example, in a complex J2EE app with multiple layers, with each
layer having its own ApplicationContext definition (in a
hierarchy), a class like SingletonBeanFactoryLocator may be used
to demand load these contexts.